Monthly to prepaid with credit remaining

Hey folks, I just purchased a prepaid SIM card at a Koodo kiosk. Currently a monthly customer, and cause I'm a keener, have a balance of about $-20. If I activate the SIM card and start prepaid, will my balance be forfeited? If that's the case, maybe I should wait til my next bill to switch to prepaid and pay off monthly bill? Thanks in advance! :D